Activation of the K-ras gene by insertion mutations in chemically induced rat renal mesenchymal tumors.

Previously we reported the detection of transforming K-ras sequences in methyl(methoxymethyl)nitrosamine (DMN-OMe)-induced rat renal mesenchymal tumors by NIH3T3 transfection assays. Subsequent analysis by selective oligonucleotide hybridization revealed a variety of activating point mutations in codon 12 of K-ras in most of these tumors and in their NIH3T3 transformants, but in some, point mutations could not be detected by this technique. In the current study, insertion mutations were detected in two DMN-OMe-induced tumors from this group with previously undefined transforming K-ras alterations. These primary tumors and their NIH3T3 transformants contained K-ras sequences with either a 9 bp or a 12 bp repeat in exon one, both of which included codon 12. No other mutations in the entire coding region of the K-ras gene were observed. Site-directed mutagenesis studies by others have determined that deletions and insertions near codon 12 can activate the ras gene, but this is the first demonstration of insertional activation of K-ras in a chemically induced rat tumor.